Method of manufacture of chalcogenide-based photovoltaic cells

ABSTRACT

The invention is a method of forming a cadmium sulfide based buffer on a copper chalcogenide based absorber in making a photovoltaic cell. The buffer is sputtered in two steps the first being at low rates or relatively high pressures and the second at high rates or relatively low pressures. The resulting cell has good efficiency and according to one embodiment is characterized by a narrow interface between the absorber and buffer layers. The buffer is further characterized according to a second embodiment by a relatively high oxygen content.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC §371 national phase filing of PCT/US Ser. No. 12/61,273 filed Oct. 22, 2012, which claims the benefit of U.S. Application No. 61/552,475, filed Oct. 28, 2011.

FIELD OF THE INVENTION

This invention relates to a method of manufacture of chalcogenide-based photovoltaic cells and particularly to a method of forming a buffer layer in such cells and the cells made by this method.

BACKGROUND OF THE INVENTION

Photovoltaic cells can be made using p-type chalcogenide based materials as absorber layers which convert incident light or radiation to electrical energy. These p-type chalcogenides are typically selenides, sulfides or sulfide selenides of at least one, and more typically at least two or three of the following metals: Cu, In, Ga, Al (referred to herein as CIS, CISS, CIAS, CIASS, CIGS, CIGSS, or CIAGSS depending upon the combination of elements used). Using a CdS based buffer layer near or adjacent to the p-type chalcogenide is also known.

It is known that CdS layers can be formed on various substrates by chemical bath deposition, physical vapor deposition or sputtering. See e.g. Abou-Ras et al (Thin Solid Films 480-481 (2005) 118-123) and U.S. Pat. No. 5,500,055. Abou-Ras specifically looked at the effect of deposition method comparing CBD deposition to PVD deposition and observed that CBD deposition of the CdS created more efficient cells compared with cells made with PVD. Abou-Ras proposes that the lack or decrease of interdiffusion at the CIGS-CdS interface in the case of PVD deposited cells is a reason for their decreased efficiencies.

SUMMARY OF THE INVENTION

Applicants have surprisingly found that sputtering buffer material such as CdS onto the p-type chalcogenide at relatively high pressures and/or relatively low rates in a substantially inert atmosphere leads to less interdiffusion with the underlying chalcogenide absorber than does sputtering at the traditional low pressure sputtering conditions but also leads to higher cell efficiencies than does the traditional low pressure sputtering. This is particularly unexpected in view of Abou-Ras's teaching about the benefit of interdiffusion. Applicants have further discovered that surprisingly the benefits of sputtering at the low rates and/or high pressure are obtained even when only a thin interfacial layer of the buffer is formed at the high pressure and the remainder of the buffer material is formed at higher rates, lower pressure or higher power conditions.

Thus, according to one embodiment, the invention is a method comprising forming a chalcogenide-based absorber layer on a substrate, forming a first portion of a buffer layer comprising cadmium and sulfur on the absorber by sputtering in an inert atmosphere at a low pressure and a rate of formation of less than 10 nm/min to form a layer having a thickness less than 20 nm, preferably less than 15 nm and at least 1 nm. Preferably this is accomplished using a working pressure of from 0.08 to 0.12 mbar (0.06 to 0.09 torr or 8-12 Pa) and/or at low sputtering power. After forming the first portion, the method further comprises forming a second portion of the buffer layer by sputtering at a rate of at least 20 nm/min to form a second portion having a thickness of at least 10 nm. Preferably, the second portion is formed at a working pressure of less than 0.05 mbar (0.04 torr or 5 Pa), preferably less than 0.01 (0.008 torr or 1 Pa), more preferably less than 0.005 mbar (0.004 torr or 0.5 Pa). However, the rate may also be increased by increasing the sputtering power. Exact power settings required for each step will vary depending upon the sputtering tool being used.

This invention is also a photovoltaic cell made by the preceding method. The cells made by this method are characterized by interdiffusion between the CdS and absorber layer. The interdiffusion interface region is defined at the absorber region by the point at which the atomic fraction of cadmium exceeds 0.05 in energy dispersive x-ray spectroscopy (EDS) scans of cross section of the cell and defined at the buffer region by the point at which the atomic fraction of indium and selenium is less than 0.05 and preferably the atomic fraction of copper is less than 0.10. The atomic fraction is based on total atomic amounts of copper, indium, gallium, selenium, cadmium, sulfur, and oxygen. The grain size of the cadmium sulfide grains preferably is less than 50 nm, more preferably less than 30 nm, and most preferably less than 20 nm. Thus, according to one embodiment the invention is a photovoltaic cell comprising a backside electrode (also referred to herein as a backside electrical contact or backside electrical collector), a chalcogenide-based absorber in contact with the backside electrode, a cadmium sulfide based buffer layer on the absorber, a transparent conductive layer located at the opposite side of the buffer layer from the absorber layer, an electrical collector on the transparent conductive layer, wherein the cell has an interface between the absorber and the buffer of less than 10 nm thickness and the buffer preferably has an average grain size of less than 50 nm.

According to another embodiment this invention is a photovoltaic cell comprising a backside electrode, a chalcogenide based absorber in contact with the backside electrode, a cadmium sulfide based buffer layer on the absorber, a transparent conductive layer located at the opposite side of the buffer layer from the absorber layer, an electrical collector on the transparent conductive layer, wherein the atomic fraction of oxygen in the first portion of the cadmium sulfide based buffer layer is at least 0.20 and in the second portion is less than 0.20, preferably less than 0.15, more preferably less than 0.10.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a cross section of a representative cell according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of this invention includes forming a chalcogenide based absorber layer (absorbs electromagnetic radiation at relevant wavelengths and converts to electrical energy) on a substrate and then forming a cadmium sulfide buffer layer on that absorber. When the method is used to form a photovoltaic device, additional layers as are known in the photovoltaic arts also will typically be added. For example, the substrate will typically include or bear backside electrical contacts. A transparent conductive layer will be found above the buffer and an electrical collector system (e.g. a grid) will typically be located above the transparent conductive layer. An optional window layer may be used and protective layers may be applied over the transparent conductive layer and/or the electrical collector. With the exception of the cadmium sulfide based buffer layer, these additional layers may be formed by any method known in the art.

FIG. 1 shows one embodiment of a photovoltaic article 10 that may be made by processes of the invention. This article 10 comprises a substrate incorporating a support 22, a backside electrical contact 24, and a chalcogenide absorber 20. The article 10 further includes a buffer region 28 incorporating an n-type chalcogenide composition of the present invention, an optional front side electrical contact window region 26, a transparent conductive region 30, a collection grid 40, and an optional barrier region 34 to help protect and isolate the article 10 from the ambient. The buffer region has an interfacial area 28 a sputtered at relatively high pressure and an additional region 28 b formed at lower pressure. Each of these components is shown in FIG. 1 as including a single layer, but any of these independently can be formed from multiple sublayers as desired. Additional layers (not shown) conventionally used in photovoltaic cells as presently known or hereafter developed may also be provided. As used occasionally herein, the top 12 of the cell is deemed to be that side which receives the incident light 16. The method of forming the cadmium sulfide based layer on the absorber can also be used in tandem cell structures where two cells are built on top of each other, each with an absorber that absorbs radiation at different wavelengths.

The support 22 may be a rigid or flexible substrate. Support 22 may be formed from a wide range of materials. These include glass, quartz, other ceramic materials, polymers, metals, metal alloys, intermetallic compositions, paper, woven or non-woven fabrics, combinations of these, and the like. Stainless steel is preferred. Flexible substrates are preferred to enable maximum utilization of the flexibility of the thin film absorber and other layers.

The backside electrical contact 24 provides a convenient way to electrically couple article 10 to external circuitry in cases where support 22 is non-conductive. Contact 24 may be formed from a wide range of electrically conductive materials, including one or more of Cu, Mo, Ag, Al, Cr, Ni, Ti, Ta, Nb, W, combinations of these, and the like. Conductive compositions incorporating Mo are preferred. The backside electrical contact 24 may also help to isolate the absorber 20 from the support 22 to minimize migration of support constituents into the absorber 20. For instance, backside electrical contact 24 can help to block the migration of Fe and Ni constituents of a stainless steel support 22 into the absorber 20. The backside electrical contact 24 also can protect the support 22 such as by protecting against Se if Se is used in the formation of absorber 20.

The chalcogenide absorber 20 preferably incorporates at least one p-type Group 16 chalcogenide, such as Group 16 selenides, sulfides, and selenides-sulfides that include at least one of copper, indium, and/or gallium. In many embodiments, these materials are present in polycrystalline form. Advantageously, these materials exhibit excellent cross-sections for light absorption that allow absorber 20 to be very thin and flexible. In illustrative embodiments, a typical absorber region 20 may have a thickness in the range from about 300 nm to about 3000 nm, preferably about 1000 nm to about 2000 nm.

Representative examples of such p-type chalcogenides absorbers are selenides, sulfides, tellurides, and/or combinations of these that include at least one of copper, indium, aluminum, and/or gallium. More typically at least two or even at least three of Cu, In, Ga, and Al are present. Sulfides and/or selenides are preferred. Some embodiments include sulfides or selenides of copper and indium. Additional embodiments include selenides or sulfides of copper, indium, and gallium. Aluminum may be used as an additional or alternative metal, typically replacing some or all of the gallium. Specific examples include but are not limited to copper indium selenides, copper indium gallium selenides, copper gallium selenides, copper indium sulfides, copper indium gallium sulfides, copper gallium sulfides, copper indium sulfide selenides, copper gallium sulfide selenides, copper indium aluminum sulfides, copper indium aluminum selenides, copper indium aluminum sulfide selenide, copper indium aluminum gallium sulfides, copper indium aluminum gallium selenides, copper indium aluminum gallium sulfide selenide, and copper indium gallium sulfide selenides. The absorber materials also may be doped with other materials, such as Na, Li, or the like, to enhance performance. In addition, many chalcogenide materials could incorporate at least some oxygen as an impurity in small amounts without significant deleterious effects upon electronic properties.

One preferred class of CIGS materials may be represented by the formula Cu_(a)In_(b)Ga_(c)Al_(d)Se_(w)S_(x)Te_(y)Na_(z)  (A) Wherein, if “a” is defined as 1, then:

-   “(b+c+d)/a”=1.0 to 2.5, preferably 1.0 to 1.65 -   “b” is 0 to 2, preferably 0.8 to 1.3 -   “c” is 0 to 0.5, preferably 0.05 to 0.35 -   “d” is 0 to 0.5, preferably 0.05 to 0.35, preferably d=0 -   “(w+x+y)” is 2 to 3, preferably 2 to 2.8 -   “w” is 0 or more, preferably at least 1 and more preferably at least     2 to 3 -   “x” is 0 to 3, preferably 0 to 0.5 -   “y” is 0 to 3, preferably 0 to 0.5 -   “z” is 0 to 0.5, preferably 0.005 to 0.02

The absorber 20 may be formed by any suitable method using a variety of one or more techniques such as evaporation, sputtering, electrodeposition, spraying, and sintering. One preferred method is co-evaporation of the constituent elements from one or more suitable targets, where the individual constituent elements are thermally evaporated onto a hot surface coincidentally at the same time, sequentially, or a combination of these to form absorber 20. After deposition, the deposited materials may be subjected to one or more further treatments to finalize the absorber properties.

Optional layers (not shown) may be used on the substrate in accordance with conventional practices now known or hereafter developed to help enhance adhesion between backside electrical contact 24 and the support 22 and/or between backside electrical contact 24 and the absorber region 20. Additionally, one or more barrier layers (not shown) also may be provided over the backside of support 22 to help isolate device 10 from the ambient and/or to electrically isolate device 10.

The buffer region 28 is cadmium sulfide based material deposited in two steps on the absorber 20. The first portion 28 a is formed by sputtering at a rate of less than 10 nm/min, preferably less than 8 nm/min and most preferably less than 5 nm/min to form a layer having a thickness less than 20 nm, preferably less than 15 nm and preferably at least 1 nm, more preferably at least 5 nm or by sputtering using a working pressure of at least 0.08 millibar (0.06 torr, 8 Pa), more preferably at least 0.09 millibar (0.067 torr, 9 Pa), and most preferably about 0.1 millibar (0.075 torr, 10 Pa) and no greater than 0.12 millibar (0.09 torr, 12 Pa), more preferably no greater than 0.11 millibar (0.083 torr, 11 Pa). Preferably, the atmosphere is inert or a sulfur containing gas, but is most preferably inert. Power settings may be adjusted to achieve the desired rate. Exact power settings required will vary depending upon the sputtering tool being used.

After forming the first portion, the method further comprises forming a second portion the buffer layer by sputtering at a rate of at least 20 nm/min to form a second portion having a thickness of at least 10 nm, preferably at least 15 nm, more preferably at least 20 nm, more preferably still at least 30 nm, but preferably no more than 200 nm, more preferably no more than 100 nm, more preferably still no more than 50 nm and most preferably no more than 30 nm. Preferably, the second portion is formed at a working pressure of less than 0.05 mbar (0.04 torr or 5 Pa), preferably less than 0.01 (0.008 torr or 1 Pa), more preferably less than 0.005 mbar (0.004 torr or 0.5 Pa). However, the rate may also be increased by increasing the power to the sputter tool. Exact power settings required for each step will vary depending upon the sputtering tool being used.

During such a deposition approach, the substrate is typically fixed to or otherwise supported upon a holder within the chamber such as by gripping components, or the like. However, the substrate may be oriented and affixed by a wide variety of means as desired. The substrate may be provided in the chamber in a manner such that the substrate is stationary and/or non-stationary during the treatment. In some embodiments, for instance, the substrate can be supported on a rotatable chuck so that the substrate rotates during the deposition. In other embodiments, the substrate may be a moving web.

One or more targets are operable provided in the deposition system. The targets are compositionally suitable to form the desired cadmium sulfide composition. For instance, to form n-type cadmium sulfide, a suitable target has a composition that includes cadmium- and sulfur-containing compounds, and is preferably 99% pure cadmium and sulfur. Alternatively, a cadmium target can be used in the presence of a sulfur-containing gas. The resulting film comprising the first and second portions is preferably at least 15 nanometers (nm), more preferably at least 20 and more preferably still at least 30 nm but preferably no more than 200 nm, still more preferably up to 100 nm, yet more preferably up to 50 nm.

While a small amount of interdiffusion in cells made by the method of this invention may occur it is significantly less than is found when the atmosphere for sputtering is lower pressure. The interdiffusion interface region is defined at the absorber region by the point at which the atomic fraction of cadmium exceeds 0.05 in energy dispersive x-ray spectroscopy scans of cross section of the cell and defined at the buffer region by the point at which the atomic fraction of indium and selenium is less than 0.05 and preferably the atomic fraction of copper is less than 0.10. The atomic fraction is based on total atomic amounts of copper, indium, gallium, selenium, cadmium, sulfur, and oxygen. The grain size of the cadmium sulfide grains preferably is less than 50 nm, more preferably less than 30 nm, and most preferably less than 20 nm. According to this embodiment, the interface region is less than 10 nm, preferably less than 8 nm in thickness.

Atomic fraction can be determined from transmission electron microscope (TEM) line scans using energy dispersive x-ray spectroscopy (EDS). Samples for TEM analysis can be prepared by focused ion beam (FIB) milling using a FEI Strata Dual Beam FIB mill equipped with a Omniprobe lift-out tool. TEM analysis can be performed for example on a FEI Tecnai TF-20XT FEGTEM equipped with a Fischione high angle annular dark field (HAADF) scanning TEM (STEM) detector and EDAX EDS detector. The operating voltage of the TEM can be level suitable for the equipment for example voltages of about 200 keV are useful with the equipment mentioned above.

Spatially-resolved EDS line scans can be acquired in STEM mode from HAADF images. From the 100 nm long line scan, 50 spectra can be acquired using 2 nm spot-to-spot resolution and a STEM probe size of ˜1 nm. The full scale spectra (0-20 keV) can be converted into elemental distribution profiles because peak intensity (number of integrated peak counts after background removal) is directly proportional to the concentration (in weight percent) of a certain element. Weight percent is then converted to atomic percentage based on mole weight of the element. Note that as a skilled worker understands peak intensity is corrected for the detector response and the other sample-dependent factors. These adjustments are typically made based on manufacturer's parameters for their EDS equipment or other suitable reference standards. Grain size can be determined by standard analysis of TEM bright and dark field images.

The cadmium sulfide layer may contain a small amount of impurities. The cadmium sulfide layer preferably consists essentially of cadmium, sulfur, oxygen and copper. Preferably the atomic fractions of cadmium and sulfur are at least 0.3, the atomic fraction of oxygen is at least 0.2 and the atomic fraction of copper is less than 0.15, more preferably less than 0.10.

The atmosphere for sputtering is preferably an inert gas such as argon, helium or neon. The substrate may be placed at a predetermined distance from and orientation relative to the target(s). In some modes of practice, this distance can be varied during the course of the deposition, if desired. Typically, the distance is in the range from about 50 millimeters (mm) to about 100 mm. Preferably, the distance is from about 60 mm to about 80 mm. Prior to starting deposition, the chamber typically is evacuated to a suitable base pressure. In many embodiments, the base pressure is in the range from about 1×10⁻⁸ Torr to about 1×10⁻⁶ Torr.

Conveniently, many modes of practice may be carried out at a temperature in the range of from about 20° C. to about 30° C. Of course, cooler or warmer temperatures may be used to help control deposition rate, deposition quality, or the like. The deposition may be carried out long enough to provide a layer of n-type material having a desired thickness, uniformity, and/or the like.

Optional window region 26, which may be a single layer or formed from multiple sublayers, can help to protect against shunts. Window region 26 also may protect buffer region 28 during subsequent deposition of the TCO region 30. The window region 26 may be formed from a wide range of materials and often is formed from a resistive, transparent oxide such as an oxide of Zn, In, Cd, Sn, combinations of these and the like. An exemplary window material is intrinsic ZnO. A typical window region 26 may have a thickness in the range from about 1 nm to about 200 nm, preferably about 10 nm to about 150 nm, more preferably about 80 to about 120 nm.

The TCO region 30, which may be a single layer or formed from multiple sublayers, is electrically coupled to the buffer region 28 to provide a top conductive electrode for article 10. In many suitable embodiments, the TCO region 30 has a thickness in the range from about 10 nm to about 1500 nm, preferably about 150 nm to about 200 nm. As shown, the TCO region 30 is in direct contact with the window region 26, but one or more intervening layers optionally may be interposed for a variety of reasons such as to promote adhesion, enhance electrical performance, or the like.

A wide variety of transparent conducting oxides; very thin conductive, transparent metal films; or combinations of these may be used in forming the transparent conductive region 30. Transparent conductive oxides are preferred. Examples of such TCOs include fluorine-doped tin oxide, tin oxide, indium oxide, indium tin oxide (ITO), aluminum doped zinc oxide (AZO), zinc oxide, combinations of these, and the like. In one illustrative embodiment, TCO region 30 has a dual layer construction in which a first sublayer proximal to the buffer incorporates zinc oxide and a second sublayer incorporates ITO and/or AZO. TCO layers are conveniently formed via sputtering or other suitable deposition technique.

The optional electrical grid collection structure 40 may be deposited over the TCO region 30 to reduce the sheet resistance of this layer. The grid structure 40 preferably incorporates one or more of Ag, Al, Cu, Cr, Ni, Ti, Ta, TiN, TaN, and combinations thereof. Preferably the grid is made of Ag. An optional film of Ni (not shown) may be used to enhance adhesion of the grid structure to the TCO region 30. This structure can be formed in a wide variety of ways, including being made of a wire mesh or similar wire structure, being formed by screen-printing, ink-jet printing, electroplating, photolithography, and metallizing thru a suitable mask using any suitable deposition technique.

A chalcogenide based photovoltaic cell may be rendered less susceptible to moisture related degradation via direct, low temperature application of suitable barrier protection 34 to the top 12 of the photovoltaic article 10. The barrier protection may be a single layer or multiple sublayers. As shown the barrier does not cover the electrical grid structure but a barrier that covered such grid may be used instead of or in addition to the barrier shown.

EXAMPLE

A photovoltaic cell is prepared as follows. A stainless steel substrate is provided. A niobium and molybdenum backside electrical contact is formed on the substrate by sputtering. A copper indium gallium selenide absorber is formed by a 1-stage evaporation process where copper, indium, gallium and selenium are evaporated simultaneously from effusion sources onto the stainless steel substrate, which is held at ˜550° C. for ˜80 minutes. This process results in an absorber stoichiometry of about Cu(In_(0.8)Ga_(0.2))Se₂

A cadmium sulfide layer is radio frequency (rf) sputtered from a CdS target (99.9+% purity) at 160 watts in the presence of argon. The temperature of the substrate is maintained at ≦35° C., and the target-to-substrate distance is ˜90 mm. This layer is deposited in two steps, with the targeted thickness as set forth in Table 1. The first deposition on the absorber for all samples in FIG. 1 was deposited at about 0.1 mbar (10 Pa), whereas the second deposition was done at pressures varying from 0.01 down to 0.002 mbar (1 to 0.2 Pa).

On the cadmium sulfide layer, i-ZnO and Al-doped ZnO layers were deposited via rf-sputtering. A front side electrical collection grid (Ni:Al) was deposited on Al-doped ZnO via e-beam evaporation.

Cells made substantially according to the above procedure are tested for efficiency by measuring current-voltage characteristics under illumination. As shown in Table 1 cells made using a first portion formed at high pressure and a second portion at higher rate/lower pressure has nearly equivalent performance as cells having an entire buffer layer formed at high pressure but has the benefit of greatly reducing the deposition time.

TABLE 1 Overall Deposition Deposition Portion 1 Portion 2 Average Max rate, both 1 pressure 2 pressure target target Efficiency Efficiency portions Sample (mbar) (mbar) thickness thickness (%) (%) (nm/min) Comaparative1 NA 0.002 NA 15 6.96 7.18 28 Comparative 2 NA 0.002 NA 120 6.83 8.02 28 Comparative 3 ~0.1 NA 30 NA 9-10 10.61 5.6 (several samples) 4 ~0.1 0.002 15 15 8.94 9.57 9.5 5 ~0.1 0.01 15 15 7.37 9.48 9.3 

What is claimed is:
 1. A method comprising forming a chalcogenide based absorber layer on a substrate, forming a first portion of a buffer layer comprising cadmium and sulfur on the absorber by sputtering at a rate of less than 10 nm/min or by sputtering at a working pressure of from 8 to 12 Pa to create a thickness of the first portion of 1 to 20 nm, and forming a second portion of the buffer layer comprising cadmium and sulfur by sputtering at a rate of at least 20 nm/min or a working pressure of no more than 1 Pa wherein the thickness of the second portion is at least 15 nm.
 2. The method of claim 1 wherein the atmosphere is inert.
 3. The method of claim 1 wherein sputtering is from a target of cadmium and sulfur or compounds thereof.
 4. The method of claim 1 wherein the second portion is sputtered at a rate of at least 30 nm/min.
 5. The method of claim 1 wherein the second portion is sputtered at a pressure of no more than 1 Pa.
 6. The method of claim 1 wherein the first portion is sputtered at a rate of less than 8 nm/min.
 7. The method of claim 1 wherein the first portion is sputtered at a pressure of 8-12 Pa and the thickness of the first layer is 5 to 15 nm.
 8. The method of claim 1 wherein the substrate is flexible and bears a backside electrical contact.
 9. The method of claim 1 further comprising forming a transparent conductive layer over the buffer layer.
 10. The method of claim 9 further comprising forming a window layer between the transparent conductive layer and the buffer layer.
 11. The method of claim 9 comprising forming an electrical connection grid on the transparent conductive layer.
 12. The method of claim 9 comprising providing a barrier layer over the transparent conductive layer.
 13. The method of claim 11 comprising providing a barrier layer over the transparent conductive layer and the grid. 